The present invention relates to a liquid crystal display device and a method for manufacturing the same wherein the device employs columnar spacers (projections) for maintaining a gap into which liquid crystal is injected constant.
A liquid crystal display device is arranged to create images by varying an amount of light 5 from a backlight light source by controlling conditions of orientation of liquid crystal molecules through a thin film transistor (TFT) 1 as illustrated in FIG. 9. In case liquid crystal LC is employed as optical shutters, accuracies of thicknesses of liquid crystal layers (gaps) account for influences on display characteristics such as light transmission rate, contrast ratio or response speed, so that it is crucial to maintain the thickness constant. It is generally the case that this is achieved through bead-like minute particles called spacers 4. The spacers 4 are composed of spherical silica or divinyl benzene polymer.
Spacer dispersing processes and liquid crystal injecting processes performed thereafter will now be explained.
An orientation film is formed on a TFT substrate 2 which is build-in with a TFT 1 or electrode wiring, and this substrate undergoes rubbing treatments (a surface of the orientation film is rubbed with a cloth). Thereafter, spacers 4 are dispersed onto the TFT substrate 2, and the substrate 2 is stack joined with a color filter substrate 3 to be remote therefrom by a specified gap. It should be noted that, for instance, a dry dispersing method is actually used for dispersing a specified amount of bead-like spacers 4 over the entire substrate in an uniform manner.
It should further be noted that peripheries of both substrates are sealed by using a sealing material except of an injection inlet through which liquid crystal is injected. This substrate is put into a vacuum chamber. When a pressure within the both substrates becomes not more than a specified pressure in the vacuum chamber through decompressing actions, the liquid crystal injection inlet is immersed into a liquid crystal bath, and the vacuum chamber is returned to atmospheric pressure. Owing to a pressure difference between the atmospheric pressure and the internal pressure of the both substrates (decompressed state), liquid crystal is filled into the gap.
However, many technical drawbacks such as degradations in uniformity of the gap owing to aggregation of spacers, increases in tact caused in spacer dispersing processes, and leakage of light owing to blurs in orientation in proximities of spacers were pointed out in view of such spacer dispersing methods.
Especially in liquid crystal display devices of birefringence mode which are represented by those employing a transverse field method, specifications for setting gaps are quite strict (designed value ±0.1 μm), and blurs in spacer dispersion (agglomerated blurs etc.) remarkably degrade display qualities when using the transverse field method. Even if it is possible to disperse the spacers in an uniform manner, the presence of variations in spacer diameters (average value ±0.2 μm) make it difficult to neglect variations in the spacers themselves.
In order to solve the above problem, it has been proposed for a columnar spacer method in which a film formed on a glass substrate (such as a color filter layer) is remained through etching to build-in projections onto the glass substrate. Since it is possible to perform selective disposition, no blurs in dispersing columnar spacers will be generated and it is further possible to exhibit uniformity of spacer heights for defining the gap since precise film forming processes are employed. Influences of blurs in orientation might be also be eliminated owing to the fact that columnar spacers can be selectively formed in a light-shielding region. It is disclosed, for instance, in Japanese Unexamined Patent Publication No. 186379/1998 to manufacture separate projections by using a light-shielding film, and Japanese Unexamined Patent Publication No. 268356/1998 discloses that separate projections are manufactured by using a color filter film. Optimization of shapes of columnar spacers or methods for disposing the same are further introduced in Japanese Unexamined Patent Publication No. 73088/1997 or Japanese Unexamined Patent Publication No. 48636/1998 for the purpose of decreasing mechanical damages on the columnar spacers caused through a cloth for rubbing the orientation film or of improving blurs in rubbing in proximities of spacers.
However, in putting the above techniques related to columnar spacers into practical use, there was presented a new technical drawback that ambient temperatures need to be considered. These drawbacks will now be explained with reference to the drawings.
(1) Display blurs owing to thermal expansion of liquid crystal (see FIG. 10).
After performing vacuum injection, the liquid crystal LC is maintained in a negative pressure condition (wherein P1 is not more than atmospheric pressure), and the columnar spacers 6 receive compressive pressure p from substrates 2, 3 corresponding to a difference in internal pressure P1 of the liquid crystal and atmospheric pressure as illustrated in FIG. 10 whereupon the columnar spacers 6 are elastically deformed (elastic compression amount F). However, since the liquid crystal LC is heated through radiant heat from a light source or a driving circuit, the liquid crystal LC thermally expands so that its internal pressure P1 is increased. Consequently, when the internal pressure P1 of liquid crystal LC exceeds the atmospheric pressure to become P2, the columnar spacers 6 will peel off from the glass substrate 3 which had been supported thereby and a substrate supporting effect of the columnar spacers 6 will vanish. Effects of the spacers of applying uniform pressure to the substrate might not be exhibited and display blurs accompanying degradations in uniformity of the gap will be induced.
While values of thermal expansion are dependent on liquid crystal materials, the value of thermal expansion of liquid crystal having, for instance, a thermal expansion coefficient of α=7.46×10−4/K is estimated to be approximately 0.075 μm in case of a temperature increase of 20° C. from room temperature (approximately 20° C.), provided that an initial gap satisfies g=5 μm.
(2) Generation of bubbles owing to negative pressure of liquid crystal (see FIG. 11)
Owing to balancing conditions of force, the columnar spacers 6 will push the glass substrate 3 up at a reaction force f1 corresponding to a difference in internal pressure P1 of the liquid crystal LC and atmospheric pressure (see FIG. 11(a)). In case heat shrinkage of the liquid crystal LC occurs accompanying a decrease in temperature of the liquid crystal LC, an elastic shrinkage amount of the columnar spacers 6 will increase following the shrinkage. This increase in elastic shrinkage amount will cause an increase in reaction force f2 of the columnar spacers 6 as well. Simultaneously therewith, the difference in pressure between atmospheric pressure and internal pressure P3 of the liquid crystal LC will increase to maintain balance with the reaction force f2. The internal pressure of liquid crystal will accordingly decrease to promote conversion of the liquid crystal LC to a negative pressure condition and to generate bubbles owing to carburetion of liquid crystal.
It is thus an object of the present invention to solve the above-described problems, and to provide a liquid crystal display device and a method for manufacturing the same capable of preventing generation of display blurs owing to thermal expansion of liquid crystal and generation of bubbles owing to heat shrinkage.